Portable devices such as, for example, digital cameras or mobile phones offer integrated flash light capability for illuminating the scenery from which a picture is to be taken. To provide a slim form factor alternative to a XENON camera flash light, light sources such as white LEDs can be used. In order to become a usable flash light alternative, one or multiple LEDs must be driven at power levels several orders of magnitude above the maximum current capability of the system's battery. Beside the flash light functionality and the high currents consumed by the LEDs, there are other components and applications of mobile electronic devices that tend to consume similarly high currents. Therefore, with other loads as well, the battery loading limitations are relevant.
The invention provides an electronic device, which includes a DC-DC power converter for converting a primary supply voltage into an output voltage at an output node. The DC-DC converter is adapted to be coupled to a super capacitor (i.e. a capacitor with a very large capacitance value) at the output node. A control stage is provided for operating the regulated DC-DC converter, in a forward direction in a first mode (e.g. a boost mode), providing a first voltage level (e.g. a boost voltage level) at the output node. The control stage is further adapted to operate the regulated DC-DC converter also in a reverse direction in a second mode (e.g. a buck mode) providing a second voltage level (e.g. a buck voltage level) at an auxiliary node. The auxiliary node is arranged between a primary voltage supply providing the primary supply voltage and the output node. Furthermore, the control stage is adapted to control the DC-DC converter to provide a current to the auxiliary node in the second mode (in reverse direction), using the super capacitor as a power source. During the first mode of operation (i.e. in a forward direction), the power stage DC-DC converter operates as a boost converter, thereby maintaining the main output voltage at a regulated constant voltage rail. The super capacitor is biased with the same potential as the output voltage rail. Then during a second mode of operation (i.e. in reverse or backward direction), the operation of the converter is reversed into buck mode for supplying a current to the auxiliary node from the super capacitor. In the second mode, the voltage level at the auxiliary node can be lower (i.e. for example a lower or buck voltage level) than in the first mode. Preferably, in the second or buck mode (i.e. in reverse direction), the DC-DC converter is controlled by a control stage so that a high instantaneous current is provided to the auxiliary node from the super capacitor. According to the invention, the same components of the DC-DC converter (e.g. switches, inductor etc.) are used in a forward direction as a boost converter and in a reverse direction as buck converter. Input and output nodes are swapped, so that the output node in the forward direction becomes the input node in the reverse direction (second mode). Therefore, the node to which the load is coupled, while the DC-DC converter is operated in the forward direction, becomes the power supplying node when the DC-DC converter is switched into the second mode (i.e., in reverse direction). The auxiliary supply mechanism in reverse direction, where the super capacitor is used as a power source allows power losses, thermal stresses and also the size of the integrated circuit to be minimized. During the second mode (i.e. when the DC-DC converter is operated in reverse direction), all of the instantaneous power (e.g. peak current) is provided by the super capacitor and the battery is not subject to high pulse loading. Therefore, a maximum amount of instantaneous current can be provided without the need for a battery that is capable of delivering the high instantaneous current. The DC-DC converter of the invention therefore provides the advantage that the battery peak loading is minimized.
The high instantaneous current in reverse mode can preferably be used for supplying a light-emitting semiconductor device, which may be coupled to the auxiliary node in series with an active current regulator. If the current is sufficiently high the light-emitting semiconductor device can be driven to produce a flash strobe. Due to the DC-DC converter's dual mode capability (i.e., there is a first mode where the DC-DC converter is driven in forward direction and a second mode where the DC-DC converter is driven in reverse direction), the high instantaneous current can be provided by the super capacitor rather than by the battery. This is particularly useful, since using a light-emitting semiconductor device to generate a flash light requires a level of power that can be several orders of magnitude above the maximum current capability of the battery used to provide the primary power supply voltage in a portable device. However, since a maximum amount of current is provided to the auxiliary node by the super capacitor during the second mode of operation (buck mode), this maximum current can be sufficient to allow, for example, a flash light to be generated by the light-emitting semiconductor device when supplied to it.